1. Field of the Invention
The present invention relates to a method of manufacturing an electrolytic capacitor for use in various types of electronic devices.
2. Background Art
As an electronic device is used at higher frequencies, an electrolytic capacitor, i.e. one of the electronic components of the electronic device, is required to be more excellent in equivalent series resistance (hereinafter referred to as ESR) characteristics in a higher-frequency range and have larger capacitance. In recent years, to reduce the ESR in the high frequency range, studies have been made on a solid electrolytic capacitor that uses a conductive polymer having high electric conductivity as a solid electrolyte thereof. In order to satisfy the demand for larger capacitance, a solid electrolytic capacitor is produced, which having a wound structure in which an anode foil and a cathode foil are wound with a separator interposed therebetween to form a capacitor element and a conductive polymer is incorporated into the capacitor element.
Generally known methods of forming a conductive polymer inside the capacitor element in this wound solid electrolytic capacitor include chemical oxidative polymerization of 3,4-ethylene dioxythiophene monomers with p-ferric toluenesulfonate, and chemical oxidative polymerization of pyrrole monomers with ferric chloride or persulfate. In these methods, the capacitor element is impregnated with reactants for an in-situ reaction.
In another wound electrolytic capacitor proposed, both of a solid electrolyte composed of a conductive polymer, and an electrolytic solution are used as derivation materials of the cathode. Also in this case, similar to the above solid electrolytic capacitor, a monomer, oxidizing agent, and dopant impregnated into the capacitor element are used for chemical oxidative polymerization in forming a conductive polymer.
In still another electrolytic capacitor proposed, a conductive polymer is prepared in the separator to form a capacitor element, and an electrolytic solution is impregnated into the capacitor element.
However, for such a conventional wound solid electrolytic capacitor, in order to form a conductive polymer inside the capacitor element, reactants such as a monomer, dopant, and oxidizing agent are impregnated into the capacitor element and are reacted in situ. Thus, it is difficult to control the reactions uniformly in the capacitor element. Therefore, uniformly forming the conductive polymer requires a method of making plurality times of reaction. This complicates the process and increases the production cost.
Furthermore, the in-situ chemical reactions generate reaction by-produces and unreacted portions, and necessitate a cleaning process for removing these substances. This cleaning process needs be performed at every time of the reaction, resulting in one of the factors in the cost increase caused by the complicated process.
In addition, because a conductive polymer poor at recovering a dielectric oxide film (hereinafter referred as “oxide film”) is used, it is difficult to form a capacitor having high withstand voltage. The obtained capacitor has a rated voltage as low as 25 to 30 V. Even in this rated voltage range, leakage current accidentally may increase or a defect in the oxide film may cause a short-circuit failure during use. The general method of improving the short-circuit resistance is to thicken the oxide film and thus increase the withstand voltage. However, in this case, as the thickness of the oxide film increases, the capacitance decreases. Thus, the capacitance per unit volume of the capacitor considerably decreases.
Furthermore, because sodium persulfate and p-ferric toluenesulfonate, i.e. strong acids, are used as a dopant and oxidizing agent, complete removal of the impurities inside the capacitor is difficult even with the cleaning process. Because these impurities adversely affect the oxide film as strong acids, the withstand voltage is lowered. Further, in a high humidity environment, the strong acids can be liberated in water and corrode each member. For these reasons, ensuring sufficient reliability is difficult.
In one of the electrolytic capacitors proposed, both of a solid electrolyte formed of a conductive polymer and an electrolytic solution are used as cathode derivation materials. However, the method of forming the conductive polymer is the same in the case of the above wound solid electrolytic capacitor. Thus impurities of a dopant and oxidizing agent that affect the withstand voltage and corrosiveness are present, and the electrolytic solution facilitates diffusion of the impurities inside the capacitor element. These phenomena considerably decrease the reliability. In addition, when both conductive polymer and electrolytic solution are used, a phenomenon in which a dopant contained in the conductive polymer leaches into the electrolytic solution, so-called a dedoping reaction, considerably decreases the electric conductivity. Thus the reliability is decreased. Further, the conductive polymer is polymerized by chemical reactions inside the capacitor element. These reactions form the conductive polymer in defective portions in the oxide film, which reduces the ESR. However, the effect in which the electrolytic solution recovers the defective portions in the oxide film is not exerted. Thus enhancing the withstand voltage is difficult.
In the electrolytic capacitor using a conductivity-imparted separator and an electrolytic solution, conductivity is imparted only to the separator. Thus the contact resistance to the anode and cathode foils is large, and the effect of reducing the ESR is small.